1. Field of the Invention
The present invention relates to a hydraulic twin-tube type shock absorber and specifically relates to an inverted twin-tube type shock absorber in which a piston-rod extends downward from a piston.
2. Description of the Prior Art
A hydraulic shock absorber composes one member of a suspension system for a vehicle and functions to damp the bound/rebound motion of a vehicle. Two typical types of hydraulic shock absorbers are known. One is a twin-tube type shock absorber, one example of which is disclosed in Japanese Pat. No. SHO 53-70278. The other type of shock absorber is a mono-tube type shock absorber, one example of which is disclosed in Japanese Pat. No. SHO 55-132432. The mono-tube type shock absorber of SHO 55-132432 is called D'carbon and SHO 55-132432 discloses an inverted use of a mono-tube type shock absorber in which a piston-rod thereof extends downward from a piston. Japanese Utility Model Publication No. SHO 61-14676 discloses another example of the inverted mono-tube type shock absorber, though the feature of the mono-tube type shock absorber of SHO 61-14676 is in the dust cover thereof. Since the mono-tube type shock absorber utilizes high pressure gas (e.g., a gas pressurized up to 20-30 kg/cm.sup.2) to fill one portion of the reservoir chamber thereof, contact pressure between the piston-rod and an oil seal therefor which receives the high pressure of the gas via oil located in the chamber is very high and thus it is difficult to obtain smooth movement of the piston-rod. Conversely, movement of the piston-rod of a twin-tube shock absorber is smooth, because the twin-tube type shock absorber utilizes gas of low pressures. The present invention relates to the shock absorber of the former type, that is, of the twin-tube type for the purpose of obtaining smooth movement of the piston-rod.
For the purpose of better understanding of the features of the twin-tube type shock absorber of the present invention, problems in a conventional twin-tube type shock absorber will be explained referring to FIG. 6. As shown in FIG. 6, a conventional twin-tube type shock absorber comprises an outer shell 1 and a cylinder 3 disposed within the outer shell 1. A space defined between the outer shell 1 and cylinder 3 is filled with gas 11 and oil 12 and composes a reservoir chamber 13. The reservoir chamber 13 is separated from a space inside of the cylinder 3 by a base valve which is fixed to lower end portion of the cylinder 3. The base valve 2 has a damping port 7 and a return port 9 and oil flowing through the damping port 7 generates a damping force under a compression stroke of the shock absorber. A piston 14 which is slidably inserted in the cylinder 3 divides the space inside the cylinder 3 into two chambers. The piston has a damping port 8 and a return port 10, and the oil flowing between the two chambers through the damping port 8 generates a damping force under a tension stroke of the shock absorber. A piston-rod 6 which is connected to the piston 15 penetrates a rod guide 4 connected to an upper end portion of the cylinder 3 and an upper end plate 5 of the outer shell 1. An upper end portion of the piston-rod 6 is connected to a portion of the body of a vehicle.
The conventional twin-tube type shock absorber of FIG. 6 has the following problems.
First, the surface of the oil in the reservoir chamber 13 is easily inclined due to an inertia force acting on the oil and the gas 11 in the reservoir chamber 13 can easily flow into the chamber inside the cylinder 3. To prevent the gas from flowing from the reservoir chamber 13 into the chamber inside cylinder 3, it is necessary to provide a special check seal mechanism at the base valve 2.
Second, since the oil in the reservoir chamber 13 is vibrated by the outer shell 1 which is connected to wheel side members, the surface of the oil bubbles to a great extent corresponding to severe vertical movements of wheels, and the bubbles of gas will easily flow into the chamber inside the cylinder 3. To prevent the bubbles of gas from flowing into the chamber inside cylinder 3, it is necessary to provide a special means for suppressing bubbling of the surface of the oil, such as a free piston or some bubble suppressing material mixed in the oil.
Third, since the oil seal 14 is exposed directly to the gas 11 in the reservoir chamber 13, the gas 11 in the reservoir chamber 13 gradually leaks outside the shock absorber during the life of the shock absorber through the portion where the piston-rod 6 extends through the oil seal 14, whereby the surface of the oil is gradually lowered and the damping abililty of the shock absorber is gradually decreased. To prevent such gas leakage, it becomes necessary to provide a special check seal mechanism, such as a special oil collecting means, in the seal member 14 itself.
Fourth, since a considerably large area of the surface of the outer shell 1 contacts the gas 11 in the reservoir chamber 13, the heat disipating ability of the shock absorber is not good and deterioration of the oil will easily occur.
Finally, since the entire portion of the shock absorber excluding the piston-rod 6 is connected to a member which is disposed on the wheel side of a suspension spring of the suspension system, the mass of vehicle members on the wheel side of the suspension spring is increased and the vibrational characteristics of the vehicle body are negatively affected. In addition, as previously mentioned, since the outer shell 1 of the shock absorber directly receives rapid vertical movements of the wheels, the oil is vibrated by the outer shell 1 and bubbling at the surface of the oil is increased.